Many people have astigmatic aberrations in the corneas of their eyes. To help correct this optical error, toric contact lenses are commonly provided. Conventional toric lenses have a central optical zone and a peripheral zone. The surface of the central optical zone is toric (more accurately “sphero-toric”), that is, cylindrical along a first axis and spherical along a second normal axis, with each meridian of the surface described by a spherical equation and having a single radius, with the radius being different at different meridians. And the surface of the peripheral zone is spherical with a flatter slope than the central zone surface. These sphere-based surfaces are relatively easy and inexpensive to manufacture. To further facilitate high-volume manufacturing, the central toric surface is typically formed on the posterior surface of the lens.
FIGS. 1 and 2 show a conventional prior art toric contact lens 10 with a posterior surface 12 having a central sphero-toric optical zone 14 and a peripheral spherical zone 16. These two zones 14 and 16 meet at a junction 18 that forms an oval. At this junction, the slope of the posterior surface 12 changes. This can be seen in FIG. 2, which shows all three meridians R0, R45, and R90 superimposed together, with R0 being the cylinder meridian, R90 being the sphere meridian, and R45 being the meridian halfway therebetween, and with the slopes being exaggerated for illustration purposes. As shown in this figure, the slope change is the greatest along the R0 meridian and smallest along the R90 meridian. Along all meridians, the slope of the posterior surface 12 changes at the junction 18 from steep in the central zone 14 to flat in the peripheral zone 16. This steep-to-flat slope change results in a pronounced ridge at the junction 18 that bears against the cornea of the wearer. This ridge can apply pressure to the cornea, thereby causing irritation to the wearer. And for lenses made of relatively stiff materials such as silicon-hydrogel, this pressure can result in corneal epithelial staining.
Thus it can be seen that a need exists for improvements to toric lenses to provide an improved fitting relationship of the lens to the cornea for improved comfort and eye health, without sacrificing visual acuity. It is to the provision of such solutions that the present invention is primarily directed.